1. Field of the Invention
The present invention relates to a solid state image pickup device to be used as an area image sensor and its driving method, and more particularly to a solid state image pickup device suitable for use as an area image sensor of an electronic still camera, and its driving method.
2. Description of the Related Art
After mass production techniques for charge-coupled devices (CCD) have been established, video cameras, electronic still cameras and the like utilizing CCD type solid state image pickup devices as area image sensors are prevailing rapidly. CCD type solid image pickup devices are classified into several types, depending upon their structures. One type is an interline transfer type solid state image pickup device (this solid state image pickup device is hereinafter described as xe2x80x9cIT-CCDxe2x80x9d in abbreviation).
IT-CCD has a number of photoelectric conversion elements disposed along a plurality of columns and rows at a constant pitch. Each photoelectric conversion element column is constituted of a plurality of photoelectric conversion elements, and each photoelectric conversion element row is also constituted of a plurality of photoelectric conversion elements.
A number of photoelectric conversion elements each made of a p-n photodiode are formed, for example, by forming a p-type well in a desired principal surface of a semiconductor substrate and forming n-type regions (n-type impurity doped regions) having a desired shape in the p-type well as many as the number of photoelectric conversion elements to be formed. If necessary, a p+-type region (p+-type impurity doped region) is formed on each n-type region. Signal charges are accumulated in the n-type region. The n-type region functions as a signal charge accumulation region.
In this specification, the term xe2x80x9cphotoelectric conversion elementxe2x80x9d is used in some cases to mean only the signal charge accumulation region. In this specification, it is assumed that xe2x80x9cadjacent to the photoelectric conversion elementxe2x80x9d means xe2x80x9cadjacent to the signal charge accumulation region constituting the photoelectric conversion elementxe2x80x9d and that xe2x80x9ccontiguous to the photoelectric conversion elementxe2x80x9d means xe2x80x9ccontiguous to the signal charge accumulation region constituting the photoelectric conversion elementxe2x80x9d.
A charge transfer channel is formed adjacent to each photoelectric conversion element column. IT-CCD has a plurality of charge transfer channels. Each charge transfer channel is used for transferring signal charges accumulated in all photoelectric conversion elements of the photoelectric conversion element column adjacent to the charge transfer channel.
A plurality of transfer electrodes traversing in plan view each charge transfer channel are formed on an electric insulating film over the surface of the semiconductor substrate. A cross area in plan view between each transfer electrode and the charge transfer channel functions as one charge transfer stage. A vertical transfer CCD is therefore formed by the channel transfer channel and transfer electrodes.
In this specification, a charge transfer stage forming region of each transfer electrode constituting the vertical transfer CCD is called a xe2x80x9ctransfer path forming areaxe2x80x9d.
Generally, each vertical transfer CCD of an interlace drive type IT-CCD has two charge transfer stages per one photoelectric conversion element. Generally, each vertical transfer CCD of an all-pixel read type IT-CCD has three or four charge transfer stages per one photoelectric conversion element. One IT-CCD has vertical transfer CCDs same in number as the number of photoelectric conversion element columns formed in IT-CCD.
Each photoelectric conversion element photoelectrically converts incidence light into signal charges and stores the charges. The signal charges stored in each photoelectric conversion element are read to the corresponding charge transfer channel at a predetermined timing.
For a read control of signal charges from the photoelectric conversion element to the charge transfer channel, a readout gate region is formed adjacent to each photoelectric conversion element on the surface of the semiconductor substrate. This readout gate region has generally a conductivity type opposite to that of the photoelectric conversion element and charge transfer channel, in order to form a potential barrier relative to the signal charges. Each readout gate region is also adjacent to a predetermined region of the charge transfer channel.
A readout gate electrode structure is formed on the readout gate region. The readout gate electrode structure is constituted of a partial region of the transfer path forming area of a predetermined transfer electrode constituting the vertical transfer CCD. As a high voltage is applied to the readout gate structure to remove the potential barrier in the readout gate region, signal charges accumulated in the photoelectric conversion element can be read to the charge transfer channel.
Signal charges read to each charge transfer channel are transferred to an output transfer path by each vertical transfer CCD constituted of the charge transfer channel. The output transfer path is generally made of CCD (this CCD is called in some cases a xe2x80x9chorizontal transfer CCDxe2x80x9d).
The output transfer path made of the horizontal transfer CCD has N charger transfer stages per one vertical transfer CCD. Each charge transfer stage has usually one potential barrier and one potential well. In this case, N=2. If the charge transfer stage has a uniform potential, then N=3 or larger.
The output transfer path sequentially transfers the received signal charges along a lengthwise direction of the photoelectric conversion element row (this direction is hereinafter simply called a xe2x80x9crow directionxe2x80x9d), to an output unit. Similar to the vertical transfer CCD, the output transfer path is formed on the semiconductor substrate.
The vertical transfer CCD and horizontal transfer CCD have the photoelectric conversion function similar to photodiodes. In order to avoid unnecessary photoelectric conversion by the vertical transfer CCD and horizontal transfer CCD, a light shielding film is formed on an area from a photosensitive area with photoelectric conversion elements to the horizontal transfer CCD area. The light shielding film has an opening with a predetermined shape formed on each photoelectric conversion element (photodiode). An opening is formed for each photoelectric conversion element. Generally, the inner edge of the opening is inner in plan view than the outer edge, in plan view, of the signal charge accumulation region of the photoelectric conversion element.
A pixel is constituted of: one photoelectric conversion element; one readout gate region formed contiguous to the photoelectric conversion element; one readout gate electrode structure covering in plan view the readout gate region; and two to four charge transfer stages (two to four charge transfer stages on the vertical transfer CCD) associating to the photoelectric conversion element. The area of the photoelectric conversion element exposed in plan view in the opening functions as a light receiving area.
The shape and area of the light receiving area of the pixel of IT-CCD are therefore substantially determined by the shape and area, in plan view, of the opening formed in the light shielding film.
The performance such as a resolution and sensitivity of IT-CCD widely prevailing nowadays is desired to be improved further.
The resolution of IT-CCD depends largely on the pixel density. The higher the pixel density, the resolution is easier to be improved. The sensitivity of IT-CCD depends largely on the area of the light receiving area of each pixel. The larger the light receiving area of each pixel, the sensitivity is easier to be raised.
IT-CCD described in Japanese Patent Publication No. 2825702 (although this Publication has the title xe2x80x9cSolid State Image Pickup Devicexe2x80x9d, in this specification it is described as xe2x80x9cIT-CCDxe2x80x9d) has an improved pixel density while the reduction in the light receiving area of each pixel is suppressed.
This IT-CCD has a number of photoelectric conversion elements disposed along a plurality of columns and rows at a constant pitch. Each photoelectric conversion element column and each photoelectric conversion element row contain a plurality of photoelectric conversion elements. A plurality of photoelectric conversion elements constituting an even column are shifted in the column direction by about a half of the pitch between adjacent photoelectric conversion elements in each column, from a plurality of photoelectric conversion elements constituting an odd column. Similarly, a plurality of photoelectric conversion elements constituting an even row are shifted in the row direction by about a half of the pitch between adjacent photoelectric conversion elements in each row, from a plurality of photoelectric conversion elements constituting an odd row. Each photoelectric conversion element column contains photoelectric conversion elements of only the even rows or odd rows.
A vertical transfer CCD is disposed for each photoelectric conversion element column in order to transfer signal charges accumulated in the photoelectric conversion elements. The vertical transfer CCD is adjacent to the corresponding photoelectric conversion element column. Each vertical transfer CCD includes a plurality of transfer electrodes. These transfer electrodes are disposed in a honeycomb shape, in general. In each rectangular area defined by a plurality of transfer electrodes disposed in the honeycomb shape, the photoelectric conversion element is disposed in plan view.
Each vertical transfer CCD is used for transferring signal charges accumulated in all photoelectric conversion elements of the photoelectric conversion element column adjacent to the vertical transfer CCD. The vertical transfer CCD transfers the signal charges in a zigzag way to the predetermined destination.
In IT-CCD described in the above-described Publication, a number of photoelectric conversion elements and a plurality of transfer electrodes (transfer electrodes for the vertical transfer CCD) are disposed as described above to improve the pixel density while the reduction in the light receiving area of each pixel is suppressed.
In this specification, the above-described layout of a number of photoelectric conversion elements is hereinafter called a xe2x80x9cpixel-shift layoutxe2x80x9d.
For example, a xc2xd-inch, two-million-pixel IT-CCD with the pixel-shift layout used for an electronic still camera has a pixel pitch of about 2.8 xcexcm in a lengthwise direction (this direction is hereinafter called a xe2x80x9cdirection DHxe2x80x9d) of the photoelectric conversion element row. A ⅓-inch, two-million-pixel IT-CCD with the pixel-shift layout used for an electronic still camera has a pixel pitch of about 2.1 xcexcm in the direction DH.
A four-phase drive type CCD is widely used as the vertical transfer CCD, and a two-phase drive type CCD is widely used as the horizontal transfer CCD.
Pixels can be formed relatively easily at a pitch of 2.1 xcexcm in the direction DH for IT-CCD having a four-phase drive type vertical transfer CCDs and two-phase drive type horizontal CCD. However, the horizontal transfer CCD of this IT-CCD has four electrodes per one vertical transfer CCD. Namely, four transfer electrodes are formed in an area having a width of 2.1 xcexcm. The width of each transfer electrode is about 0.5 xcexcm.
In forming IT-CCD having such a horizontal transfer CCD, highly sophisticated ultra fine patterning techniques are required to make the chip size compact.
Since the horizontal transfer CCD has four transfer electrodes per one vertical transfer CCD, pulse supply terminals for supplying drive pulses to the horizontal transfer CCD have large electrostatic capacitance.
In order to raise the read frame frequency of a high resolution IT-CCD having pixels larger than two million pixels, high speed drive pulses at about 200 MHz are used for driving the horizontal transfer CCD.
Therefore, a power consumption of the horizontal transfer CCD increases, for example, to several tens mW. An increase in the power consumption results in a short battery life of a battery driven electronic still camera.
It is an object of the present invention to provide an IT-CCD and its driving method capable of improving a pixel density with ease without relying upon highly sophisticated ultra fine patterning techniques and suppressing an increase in a power consumption with ease.
According to one aspect of the present invention, there is provided a solid state image pickup device, comprising: a semiconductor substrate; a number of photoelectric conversion elements disposed on a surface of the semiconductor substrate in a plurality of columns and rows, a photoelectric conversion element column and a photoelectric conversion element row each being composed of a plurality of photoelectric conversion elements, a plurality of photoelectric conversion elements of an even column being shifted in a column direction by about a half of the pitch between adjacent photoelectric conversion elements in each photoelectric conversion element column, from a plurality of photoelectric conversion elements of an odd column, and a plurality of photoelectric conversion elements of an even row being shifted in a row direction by about a half of the pitch between adjacent photoelectric conversion elements in each photoelectric conversion element row, from a plurality of photoelectric conversion elements of an odd row; charge transfer channels each provided for two photoelectric conversion element columns and formed in the surface of the semiconductor substrate in an area in plan view between the two photoelectric conversion element columns, said charge transfer channel extending as a whole along a direction of the photoelectric conversion element column and having a zigzag shape; a plurality of transfer electrodes formed on the semiconductor substrate, traversing in plan view each of said charge transfer channels, said transfer electrode having transfer path forming areas same in number as said charge transfer channels, the transfer path forming area forming one charge transfer stage in a cross area in plan view with a corresponding charge transfer channel, adjacent two transfer electrodes with one photoelectric conversion element row being interposed therebetween repeating in plan view divergence and convergence and surrounding in plan view each photoelectric conversion element in the photoelectric conversion element row of an even or odd row to define photoelectric conversion element region, said transfer electrode extending as a whole along a direction of the photoelectric conversion element row; a readout gate region provided for each photoelectric conversion element in the surface of the semiconductor substrate to be contiguous to the photoelectric conversion element and to a corresponding charge transfer channel, said readout gate region corresponding to the photoelectric conversion element of the even row and said readout gate region corresponding to the photoelectric conversion element of the odd row being covered in plan view with different transfer electrodes; and an adjusting portion formed downstream of downstream ends of said charge transfer channels, said adjusting portion including a plurality of charge transfer stages for adjusting a phase of signal charges transferred from each of said charge transfer channels.
According to another aspect of the present invention, there is provided a driving method for a solid state image pickup device comprising: a semiconductor substrate; a number of photoelectric conversion elements disposed on a surface of the semiconductor substrate in a plurality of columns and rows, a photoelectric conversion element column and a photoelectric conversion element row each being composed of a plurality of photoelectric conversion elements, a plurality of photoelectric conversion elements of an even column being shifted in a column direction by about a half of the pitch between adjacent photoelectric conversion elements in each photoelectric conversion element column, from a plurality of photoelectric conversion elements of an odd column, and a plurality of photoelectric conversion elements of an even row being shifted in a row direction by about a half of the pitch between adjacent photoelectric conversion elements in each photoelectric conversion element row, from a plurality of photoelectric conversion elements of an odd row; charge transfer channels each provided for two photoelectric conversion element columns and formed in the surface of the semiconductor substrate in an area in plan view between the two photoelectric conversion element columns, said charge transfer channel extending as a whole along a direction of the photoelectric conversion element column and having a zigzag shape; a plurality of transfer electrodes formed on the semiconductor substrate, traversing in plan view each of said charge transfer channels, said transfer electrode having transfer path forming areas same in number as said charge transfer channels, the transfer path forming area forming one charge transfer stage in a cross area in plan view with a corresponding charge transfer channel, adjacent two transfer electrodes with one photoelectric conversion element row being interposed therebetween repeating in plan view divergence and convergence and surrounding in plan view each photoelectric conversion element in the photoelectric conversion element row of an even or odd row to define photoelectric conversion element region, said transfer electrode extending as a whole along a direction of the photoelectric conversion element row; a readout gate region provided for each photoelectric conversion element in the surface of the semiconductor substrate to be contiguous to the photoelectric conversion element and to a corresponding charge transfer channel, said readout gate region corresponding to the photoelectric conversion element of the even row and said readout gate region corresponding to the photoelectric conversion element of the odd row being covered in plan view with different transfer electrodes; and an adjusting portion formed downstream of downstream ends of said charge transfer channels, said adjusting portion including a plurality of charge transfer stages for adjusting a phase of signal charges transferred from each of said charge transfer channels, the driving method comprising the steps of: reading the signal charges accumulated in each photoelectric conversion element of at least one photoelectric conversion element row, to the charge transfer channel corresponding to the photoelectric conversion element via the readout gate region contiguous to the photoelectric conversion element, during one vertical blanking period; and converting the signal charges read to the charge transfer channel into an image signal and outputting the image signal, during a period after the one vertical blanking period and before a next vertical blanking period.
In the solid state image pickup device described above, one vertical transfer CCD is constituted of one charge transfer channel and a plurality of transfer electrodes traversing in plan view the charge transfer channels. In other words, one vertical transfer CCD is provided for two photoelectric conversion element columns. Although only one vertical transfer CCD is provided for two photoelectric conversion element columns, signal charges can be read from all photoelectric conversion elements.
With this solid state image pickup device, the number of vertical transfer CCDs necessary for reading signal charges from all photoelectric conversion elements can be reduced to a half of the number of vertical transfer CCDs of a conventional solid state image pickup device. The total number of charger transfer stages of the horizontal transfer CCD and the total number of transfer electrodes can therefore be reduced to halves of those of a conventional solid state image pickup device.
For example, a solid image pickup device having a large number of pixels of two millions can be manufactured without narrowing the width of a transfer electrode of each transfer stage of the horizontal transfer CCD. Namely, without using highly sophisticated ultra fine patterning techniques, a solid image pickup device having a large number of pixels of two millions can be manufactured.
Since the total number of charge transfer stages of the horizontal transfer CCD can be reduced to a half of that of a conventional solid state image pickup device, an increase in the electrostatic capacitance of pulse supply terminals used for supplying drive pulses to the horizontal transfer CCD can be suppressed. An increase in power consumption can therefore be suppressed easily.